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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Alexey Gladkov <legion@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Containers <containers@lists.linux.dev>
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        <87czs4u0rm.fsf@disp2133>
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Date:   Tue, 29 Jun 2021 12:09:01 -0500
In-Reply-To: <87mtr8sjvr.fsf@disp2133> (Eric W. Biederman's message of "Tue,
        29 Jun 2021 11:42:00 -0500")
Message-ID: <87a6n8simq.fsf@disp2133>
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Subject: Re: [GIT PULL] ucounts: Count rlimits in each user namespace
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ebiederm@xmission.com (Eric W. Biederman) writes:

> Linus Torvalds <torvalds@linux-foundation.org> writes:
>
>> On Tue, Jun 29, 2021 at 8:52 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>
>>> Linus Torvalds <torvalds@linux-foundation.org> writes:
>>>
>>> > Why the "sigpending < LONG_MAX" test in that
>>> >
>>> >         if (override_rlimit || (sigpending < LONG_MAX && sigpending <=
>>> > task_rlimit(t, RLIMIT_SIGPENDING))) {
>>> > thing?
>>>
>>> On second look that sigpending < LONG_MAX check is necessary.  When
>>> inc_rlimit_ucounts detects a problem it returns LONG_MAX.
>>
>> I saw that, but _without_ that test you'd be left with just that
>>
>>     sigpending <= task_rlimit(t, RLIMIT_SIGPENDING)
>>
>> and if task_rlimit() is LONG_MAX, then that means "no limits", so it is all ok.
>
> It means no limits locally.  The creator of your user namespace might
> have had a limit which you are also bound by.
>
> The other possibility is that inc_rlimits_ucounts caused a sigpending
> counter to overflow.  In which case we need to fail and run
> dec_rlimit_ucounts to keep the counter from staying overflowed.
>
> So I don't see a clever way to avoid the sigpending < LONG_MAX  test.

Hmm.  I take that back.  There is a simple clever way to satisfy all of
the tests.

- sigpending < LONG_MAX && sigpending <= task_rlimit(t, RLIMIT_SIGPENDING)
+ sigpending < task_rlimit(t, RLIMIT_SIGPENDING)

That would just need a small comment to explain the subtleties.  

Eric